Pulse-width modulated systems are used to provide high efficiency amplification and transmission in applications that vary widely from low-power consumer audio devices, such as MP3 players, to high power data transmission circuits such as base station transmitters. High efficiency is achieved by minimizing power losses due to bias current within the output stages of an amplifier. For example, in audio applications, a class-D amplifier is used to switch the terminals of a loudspeaker between two supply voltages at a frequency greater than the bandwidth of the desired output signal. Here, high frequency switching energy is filtered by the characteristics of the load circuit, for example, the inductance of the loudspeaker. Similarly, in RF applications, a power amplifier (PA), is driven by a pulse-width modulated signal with a pulse frequency greater than the bandwidth of interest. Out-of-band energy is then filtered using an RF bandpass filter, such as a SAW filter for low power applications or a cavity filter for high power applications. Because there is a minimal IR drop across the output stage of devices operating in a switched manner, as can be the case when PWM signals are used, dissipated power across the output stages of the devices are minimized and efficiency is improved.
The generation of high dynamic range PWM signals, however, poses a number of challenges at high frequencies and at radio frequencies (RF). As the bandwidths of broadband systems increase, there is a corresponding increase in the switching rate of the PWM signal used to drive the upconverter. Parasitic losses at these high switching rates, however, may lower the overall efficiency of the PWM system.
One way to reduce the switching rate in a PWM system is to use multiple parallel PWM signal levels to drive parallel RF power amplifiers. While the overall switching rate may be reduced, other inefficiencies arise such as power losses due to the loading of multiple stages, and losses due to reflected power at the bandpass filter.